Characterising Groundwater Recharge In Crystalline Aquifer Johannesburg, South Africa

Groundwater recharge assessment was undertaken in the crystalline aquifer of the Upper Crocodile River Basin, Johannesburg South Africa. The basin is characterised by the complex hydrogeological setting represented by weathered and fractured granitic gneisses overlain by quartzite, shale and dolostone. A number of recharge estimation methods including the Stable Isotope Enrichment Shift method, were tested. The measurement of δ 18O and δD in springsrevealed the presence of high elevation recharge or cold weather recharge that occurs prior to extreme evaporation, undergoing deep circulation and discharging at the contact between the Witwatersrand quartzite and the underlying shale. In the dolostones, recharge occurs after evaporation at higher elevation undergoing deeper circulation through the dissolution cavities.

The Water Table Fluctuation method in the dolostone resulted in the mean annual recharge of 99 mm/year, representing 14% of mean annual precipitation. The Reservoir Water Balance method revealed that the Pretoria Group shale aquifer contributes 16% of dam water outflow per year (groundwater discharge) which equates to 3 429 662 m3 on average, while 7% of dam inflow is lost to groundwater constituting groundwater recharge of average 2 084 131 m3 per annum. Baseflow Separation method applied gave an average recharge value of 9.4% for the entire catchment. The estimated average recharge for the entire catchment was found to be 13% corresponding to 91 mm, which equates to 374 Mm3 . The Stable Isotope Enrichment Shift Method resulted an average annual recharge of 26.1% in the aquifers composed of quartzites and 3% in the dolostones. The method is found to be promising for application in spring regimen however, a further development is recommended since small shifts exaggerate recharge while large shifts undermine it.

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